Dynamical Friction:

These formula have been applied in several situations, such as the
decay of globular cluster orbits around the Milky Way and the fate of
the nearby Magellanic clouds. But the most intriguing applications have been
in the area of galaxy cannibalism.

A small galaxy that passes through the envelope of a larger galaxy will
experience the drag force from dynamical friction. The drag will cause
the small galaxy to lose energy and spiral inward. Using the above
acceleration formula, a typical timescale can be derived such that

where the timescale has been normalized to typical galaxy values for
size and internal velocities. Since typical Coulomb logarithm values
are 3, then a small galaxy will spiral to center in a fraction of a
Hubble time.

When the small galaxy reaches the Roche limit,
it is tidally disrupted and `cannibalized'. Thus, a galaxy located at
the bottom of the cluster potential well begins to accrete mass by
cannibalizing smaller galaxies that pass through its envelope.

The following simulations shows dynamical friction at work on a dwarf
galaxy (white circle) passing near a disk galaxy.

Simulations of galaxy formation find that dynamical friction is very
efficient and that all galaxies formed from the mergers of
proto-galactic lumps that dropped out of the expanding Universe as shown
in this
galaxy formation movie.

These protogalaxies in rich environments would then evolve into clusters
of galaxies, with the growth of a central dominant member by galaxy
cannibalism as shown in this
supergiant galaxy
simulation.

So, at least in clusters of galaxies, dynamical evolution can actually
dominate the luminosity changes due to passive color evolution. And
galaxy cannibalism radically changes the shape of the luminosity
function, where the small, faint galaxies are consumed and the central
cD galaxies grows to enormous size and luminosity.

The next question is at what epoch does all this dynamical evolution
occur? Was it fixed early on as simulations suggest? or is it still
happening today? The answer lies in comparisons of the orientation of
cD galaxies with the large-scale structure of the Universe.